In recent years, the information-oriented society has continued the rapid advance and the information recording capacity exceeding 60 Gbytes has been required per 2.5-inch magnetic disk in magnetic recording apparatuses represented by HDDs (hard disk drives). In order to obtain a magnetic disk satisfying such a requirement, it is necessary to realize an information recording density exceeding 100 Gbits per inch2 (100 Gbits/inch2). For stably performing recording/reproduction with respect to such a high recording density magnetic disk, it is preferable to employ a perpendicular magnetic recording system as a magnetic recording/reproducing system.
For use in the perpendicular magnetic recording system, a CoCrPt—SiO2 (hereinafter abbreviated as CCP—SiO2) perpendicular magnetic recording medium and a CGC perpendicular medium both of which exhibit high thermal stability and excellent recording characteristics have been proposed as magnetic recording media in Non-Patent Documents 1 and 2. Herein, Non-Patent  Documents 1 and 2 are contributed by T Oikawa et al. to IEEE Transactions on Magnetics, vol. 38, 1976-1978 (2002) and by Y. Sonobe et al. to IEEE Transactions on Magnetics, vol. 37, 1667-1670 (2001), respectively. In addition, CGC is an abbreviation of Coupled Granular/Continuous and, as described in Non-Patent Document 2, this perpendicular medium is formed by a combination of an exchange-coupled continuous layer for obtaining high thermal stability and a granular CoCrPt layer for noise reduction.
Further, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-334424 (Patent Document 1), Japanese Unexamined Patent Application Publication (JP-A) No. 2003-123245 (Patent Document 2), or the like propose a perpendicular magnetic recording medium which uses a Ru—SiO2 underlayer in combination with the technique described in the foregoing Non-Patent Document 1 or 2 in order to improve the S/N ratio. In the film forming process shown in Patent Document 1 or 2, a high Ar gas pressure sputtering process is normally used for the purpose of reducing the size of magnetic grains.